1. Field of the Invention
The present invention relates to a semiconductor device used for information processing in a neural network and more particularly to an electrically reprogrammable nonvolatile memory device hereinafter referred to as an electrically plastic device which exhibits an electrically non-linear characteristic (conductivity or capacitance) in response to receiving an input energy (size or time variation).
2. Description of the Prior Art
Information processing in a neural network is known to be an excellent method for simulating the information processing of the human brain. However, it has been very difficult or almost impossible for the prior art to develop an electronic device provided with the ability the same as a synapse. This has been a big problem for the information processing devices.
A previously proposed neuron tip is classified into two groups; a semiconductor device and an optical device. The semiconductor device has a high potential for the practical realization of the neuron tip but is still not in a position to provide a plastic function suitable for the neuron tip. The previous development for the neural network has been directed to a modification of bipolar or CMOS silicon semiconductor provided with an analog operation.
On the other hand, attention has been increasingly paid to an electric field effect transistor (FET), a transconductance amplifier (OTA), a capacitor array, and a switched resistor array as possible electronic device 5 having the ability of the synapse for giving the neural network a plastic function. Among those possible devices, much attention has been directed to the FET such as a floating gate for use in EPROM and MNOS which is characterized by a low electric energy and a high gain.
An amorphous silicon has a number of irregular lattice defects and dangling bonds combined with hydrogen atoms in a different way from that of a single crystal silicon. Therefore, a molecular formula of the amorphous silicon is expressed exactly by SiH.sub.x. The amorphous silicon has another feature that it can be formed into a porous structure. Accordingly, a dopant in the amorphous silicon can move freely throughout the defective lattice and controls the electric conductivity over a wide range. The reversible movement of the dopant in the amorphous silicon is disclosed in a paper, R. Konenkamp, Solid State Communication, 73 (5), p. 323 (1990). In addition, a conduction variable device using a conductive polymer is developed for the purpose of realizing an electrically plastic device (Japanese laid-open Patent Publication Sho. 63-2000396). However, neither of the above electronic devices are provided with sufficiently satisfactory characteristic for the electrically plastic device.
There has been no available electronic device which can be used for an electronically plastic device. Moreover, the requirement for an electrically plastic device has been increased in view of the need for more efficient information processing in the near future.